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Message ID: 964     Entry time: Sat Aug 21 01:06:04 2010
Author: Zach 
Type: Laser 
Category: GYRO 
Subject: still noisy (till the day I die) 

 you can bring your crew but.. nevermind.

Alastair and I tried out the new-and-improved box #2215 on the gyro today, to see if the noise was improved at all. The short answer is "not really". There is some slight improvement at higher frequencies but at lower frequency the noise is the same or worse, depending on the gain settings we chose. 

new_box.png

I think there is something tricky about how we distribute the gain in the AOM loop (i.e. between the PDH box and the 'deviation' of the VCO), as playing with these

settings produced substantially different results in that loop, as well as in the gyro signal when it was locked.

It bothers me that the CCW (non-AOM) loop is so finicky; it would be nice to be able to achieve the many-kHz bandwidth we desire, but we are clearly limited by some resonance somewhere.

I think this should be our main priority right now, as it not only limits our sensitivity but also proves to be a huge pain in the ass when doing anything that involves touching the table.

Before we take our next breath, we should rotate the input and output mirrors 90 degrees and get a displacement noise measurement (so we can stop living in the past with the ol' 40m MZ).

After that, I would like to fully realign the gyro cavity, as well as get a reasonable modematching solution done for the CW path like there is for CCW. Then we will have decent coupling from both directions and we can tackle the servo stabilization issue.

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